1. Field of the Invention
The present invention relates to an invisible information recording medium having an invisible information recording layer above an underlying layer, the recording layer being invisible under visible light and fluorescing upon being applied with light having a particular wavelength excepting that of visible light, thus becoming visible, and an information reading apparatus for efficiently reading information from the invisible information recording medium.
2. Description of the Related Art
Such an invisible information recording medium is known as, e.g., an invisible bar code (to be referred as a stealth bar code hereinafter).
Merchandise management using bar codes is becoming popular in recent years because quickness, correctness, and simplicity of information input by means of bar codes are widely recognized. As merchandise management using bar codes is used in many fields, the following demands have arisen. For example, in some fields, a specific space for printing only the bar code cannot be kept on an underlying layer and thus a bar code must be printed on an existing printed characters of the underlying layer, and a bar code should not be printed on a part of the existing printed characters from the view-point of good appearance. In order to satisfy these demands, a stealth bar code, which is invisible under visible light and fluoresce upon being applied with light having a particular wavelength excepting that of visible light, thus becoming visible, has been proposed.
For example, a stealth bar code can be formed by printing with an ink containing a fluorescent material which is invisible under visible light but is excited upon being applied with ultraviolet rays to emit visible light, thus becoming visible.
A stealth bar code becoming visible under ultraviolet rays has following drawbacks.
In a conventional information recording medium on which a stealth bar code is printed, as shown in a sectional view of FIG. 5, an underlying printed layer 12, on which a pattern, e.g., characters, figures, and pictures, is printed, exists on an underlying layer 10 made of, for example a paper, and a stealth bar code layer 14 is printed on the underlying printed layer 12.
The stealth bar code layer 14 is formed by printing with an ink containing a fluorescent material which is invisible under visible light but is excited upon being applied with ultraviolet rays to emit visible light, thus becoming visible. When the stealth bar code layer 14 is applied with ultraviolet rays UV, it fluoresces to emit visible light, and visible light VR1 directed upward from the stealth bar code layer 14 makes visible the stealth bar code constituted by the stealth bar code layer 14.
However, since the visible light generated by fluorescence of the stealth bar code layer 14 is directed to all the directions, visible light VR2 directed downward below the stealth bar code layer 14 is reflected by the underlying printed layer 12, and reflected light VR3' decreases the contrast among the bars and spaces of the stealth bar code which has become visible. Also, a pattern, e.g., characters, figures, and pictures, of the underlying printed layer 12, which is visible with the reflected light VR3', overlaps the stealth bar code which has become visible, and decreases the contrast among the bars and spaces of the stealth bar code.
As a result, bar code information cannot sometimes be read from the visible stealth bar code with a bar code reading apparatus.
A fluorescent material is often mixed in the paper of the underlying layer 10 to make it look white. In this case, when ultraviolet rays are applied onto the stealth bar code layer 14 to read bar code information from the stealth bar code, the ultraviolet rays reach the underlying layer 10 to make the fluorescent material in the underlying layer 10 fluoresce. As a result, visible light VR3" generated by fluorescence of the underlying layer 10 decreases the contrast among the bars and spaces of the visible stealth bar code, as described above. Then, bar code information cannot sometimes be read from the stealth bar code with the bar code reading apparatus.
Although not shown, when a stealth bar code layer is directly formed on an underlying layer in which no fluorescent material is mixed, a part of the visible light generated by the fluorescence of a stealth bar code layer 14, directs downward from the visible stealth bar code layer 14 and reaches the underlying layer 10 and is reflected by it. This reflection decreases the contrast among the bars and spaces of the stealth bar code which has become visible. Then, bar code information cannot be read from the visible stealth bar code with the bar code reading apparatus.
Structures for eliminating these conventional drawbacks are disclosed in Japanese Patent Application KOKAI Publication Nos. 3-182988 and 5-12503.
According to Jpn. Pat. Appln. KOKAI Publication No. 3-182988, in order to prevent an underlying pattern from being visible when visible light generated by fluorescence of a stealth bar code layer is applied to the underlying pattern, the underlying pattern is printed by using an ink having substantially the same light reflectance as that of the surface of an underlying layer when the visible light is applied to the under laying layer.
However, when various colors of inks are used for printing the underlying pattern, it is difficult to prepare all the inks of different colors to have the reflectance described above. In addition, this proposal cannot be applied to a black ink.
According to Jpn. Pat. Appln. KOKAI Publication No. 5-12503, a layer which absorbs light having a wavelength to excite a fluorescent substance and transmits visible light is provided between a stealth bar code layer and an underlying printed layer. In this case, even when ultraviolet rays are applied onto the stealth bar code layer to read bar code information from the stealth bar code, light having the wavelength that excites the fluorescent substance does not reach the underlying printed layer, and the fluorescent substance contained in the underlying layer does not fluoresce. However, since the visible light generated by fluorescence of the stealth bar code layer reaches the underlying printed layer, the contrast of the bar code cannot be prevented from being decreased by reflected light generated when this visible light is reflected by the underlying printed layer, and by an underlying pattern which is visible with this reflected light.
A bar code reading apparatus for reading a stealth bar code applies ultraviolet rays of a single wavelength on an information recording medium on which a stealth bar code is printed by using fluorescent ink, and detects light the stealth bar code by a generated from photodetector, thereby reading data from the stealth bar code.
Further, in this kind of bar code reading apparatus, a fluorescent lamp emitting ultraviolet rays having a single wavelength is used to apply the ultraviolet rays to a bar code formed by using fluorescent ink when the bar code is read.
However, since a light emission characteristic of the fluorescent lamp cannot be stabilized until a predetermined time period have passed after a power thereof is turned on, the fluorescent lamp is continuously kept turned on. In addition, since the light emission characteristic is changed by temperature change, it is difficult to obtain a stable light emission characteristic.
When ultraviolet rays generated from the fluorescent light and having a single wavelength is applied to the stealth bar code to read bar code information from the stealth bar code, the fluorescent ink of the stealth bar code fluoresces to generate visible light. However, the visible light also applies on an underlying printed patter to make the contrast of the visible stealth bar code being decreased by a color of the underlying printed patter, and to make difficult a bar code reading by the bar code reading apparatus.
As a method for eliminating the drawbacks described above, Jpn. Pat. Appln. KOKAI Publication No. 5-149786 discloses a method for controlling a quantity of emitting light or a quantity of received light with use of an analogue-digital (A/D) converter and a digital-analogue (D/A) converter.
In addition, as a method for eliminating the influence of the color of the underlying printed pattern, Jpn. Pat. Appln. KOKAI Publication No. 3-75886 discloses a method for subtracting outputs of a sensor for detecting light reflected from the underlying printed pattern from outputs of a sensor for detecting light reflected from the stealth bar code.
A stealth bar code reading apparatus being designed such that the light emission character of the fluorescent lamp is always stable has a large power consumption, and is therefore not suitable for a portable type. However, since the light emission character of the fluorescent lamp is not stable immediately after the power of the fluorescent lamp is turned on, a method of repeatedly turning on and off the fluorescent lamp can not be used.
Although it is therefore possible to use a method for controlling a light emitting element by using an analogue-digital (A/D) converter and a digital-analogue (D/A) converter, such method requires a large circuit, and therefore, a cost for manufacturing an apparatus to perform the method is increased.
Further, in the method for eliminating influences of the color of the underlying printed patter, error of the sensor for sensing light reflected from the stealth bar code and error of the sensor for sensing light reflected from the underlying printed pattern must be considered, and thus, these two sensors increase a cost for manufacturing an apparatus to perform the method.